Novel platinum-rhodium-tungsten alloy

ABSTRACT

A NOVEL ALLOY COMPOSITION COMPRISING 25 TO 30 WT. PERCENT RHODIUM, 6 T 10 WT. PERCENT TUNGSTEN WITH THE REMAINDER BEING PLATINUM.

June 5, 1973 CHAlN L|U ETFAL NOVEL PLATINUM-RHODIUM-TUNGSTEN ALLOY Filed Feb. 15, 1972 United States Patent O 3 737,309 NOVEL PLATlNUM-RIIODIUM-TUNGSTEN ALLOY Chain T. Liu and Henry Inouye, Oak Ridge, Tenn., as-

signors to the United States of America as represented by the United States Atomic Energy Commission Filed Feb. 15, 1972, Ser. No. 226,500 Int. Cl. C22c 5/00 US. Cl. 75--172 R 4 Claims ABSTRACT OF THE DISCLOSURE A novel alloy composition comprising 25 to 30 wt. percent rhodium, 6 to wt. percent tungsten with the remainder being platinum.

CONTRACTUAL ORIGIN This invention was made in the course of, or under, a contract with the United States Atomic Energy Commission.

BACKGROUND OF THE INVENTION This invention relates generally to a novel platinum, rhodium, tungsten alloy composition, and particularly to an alloy suited for use as an encapsulation material for radioisotope fuels. Radioisotope fuels have found considerable use as both terrestrial and space power sources. Such fuels utilize an isotope which is an alpha, beta or gamma emitter. Heat is produced by stopping these nuclear emissions and converted into electrical energy by means of thermoelectric generators or thermionic converters.

The most prominent radioisotope fuels at present are PuO and Cm O These particular isotopes in the oxide form are desirable because of their refractory properties. The PuO and Cm O are sintered into cylindrical pellets 1 with a diameter of about 0.3 to 0.8 inch and a length of about 3 to 4 inches. Because of the nuclear emissions and safety requirements, each of the pellets must be contained in a double encapsulation.

Radioisotopic fuels used in space power systems must be encapsulated in a highly reliable material, not only to contain the fuel for normal operation of several years, but to survive launch abort situations, severe aerodynamic heating on re-entry, high velocity impact, and post impact environment. Prior to the invention herein disclosed, a multiple encapsulation and cladding arrangement of several alloys with one layer of alloy compensating for the shortcomings of another was considered to be the best design. This arrangement comprised multiple layers of refractory alloys clad with a platinum-rhodium alloy. However, it was found that refractory alloy components in such a system are not as reliable as desired from the standpoint of oxidation resistance and from the standpoint of compatibility of the alloys with one another and the heat source environment. The refractory alloys in such a system include T-111, composed of 8 wt. percent W-2 wt. percent Hf-balance Ta; Ta-lO W, composed of 10 wt. percent W-balance Ta; and TZM, composed of 0.5 wt. percent Ti-0.07 wt. percent Zr-balance Mo. Conventional platinum-rhodium alloys were initially considered as a protective cladding material for the refractory alloys. However, the conventional platinum-rhodium alloys, i.e., Pt20 wt. percent Rh and Pt-30 wt. percent Rh, although possessing excellent fabricability and oxidation resistance, lack the desired mechanical strength and high melting point.

SUMMARY OF THE INVENTION It is accordingly an object of this invention to provide an alloy which is fabricable, resistant to oxidation, and which has adequate strength.

Patented June 5, 1973 It is another object of this invention to provide an alloy which can be used as the entire encapsulation for an isotopic power source.

It is a still further object of this invention to provide an alloy which is compatible with radioisotope power sources.

These and other objects are accomplished by the alloy of this invention which comprises platinum, rhodium, and tungsten.

BRIEF DESCRIPTION OF THE DRAWING The single figure of drawing is a section view of a radioisotope fuel capsule according to this invention.

DETAILED DESCRIPTION The alloy of this invention comprises 25 to 30 wt. perment rhodium, 6 to 10 wt. percent tungsten with the remainder being platinum. It has been found that within these particular limits rhodium and tungsten, in combination with platinum, raise the melting point to about 2000 C. or above. All compositions within this range have a melting point between 1980 and 2050 C. EX- ceeding these compositional limits causes the alloy to become unfabricable and to lose its resistance to oxidation. An optimization of all of the desired properties of this alloy are possessed by the preferred composition which is Pt-26 wt. percent Rh-8 wt. percent W. Impurities which are common to this composition include the following elements in the represented amounts The alloys of this invention have been found to possess all of the requisite properties for a radioisotope fuel encapsulation material. Not only is this alloy compatible with the isotope heat source but it also possesses excellent oxidation resistance. With these properties the intricate cladding arrangement of the prior art can be replaced by fabricating the entire encapsulation with the alloy of this invention. The alloy of this invention is also compatible with other alloys, such as TZM and T-lll, with which it is likely to come into contact.

The figure of drawing represents the fuel capsule arrangement of this invention. The cylindrical oxide fuel pellet 1 is encapsulated in a first obround capsule 2 of the alloy of this invention. The alloy of the first capsule preferably has a thickness of about 20 mils. The second encapsulation 3, provided for safety reasons, is also constructed of the alloy of this invention and preferably has a thickness of about mils. Both encapsulations 2 and 3 are closed by means of welds 4 and end caps 5 and 6. Void spaces 7 and 8 absorb any swelling of the fuel 1. Shims may be provided in the void spaces 7 and 8 to prevent any rattling of the fuel 1 within the encapsulations. The entire capsule is about 0.5 to 1 inch in diameter and about 4 to 5 inches in length.

The alloy of this invention is prepared by either electron beam drop-casting or simple casting into pancake form. The alloy can be best fabricated by hot working in air in the temperature range of 1000 to 1250 C., fol- EXAMPLE I A 300-gram button (1 /2 inch diameter x /z inch thick) of Pt-30 wt. percent Rh-6 wt. percent W, prepared by electron beam melting was first hot forged and rolled at 1100 C. After a total of 60% reduction, the as-cast structure was broken, and the alloy was cold rolled into 25 mil sheets by progressively cold rolling and annealing at 1050" C.

EXAMPLE :I[

A ZOO-gram ingot inch diameter x 2% inch length) of Pt-26 wt. percent Rh-8 wt. percent W was prepared by electron beam drop-casting. Bar stock was obtained by hot swaging the ingot in air at temperatures of 1000 to 1200 C.

EXAMPLE III To determine the recrystallization temperature, softening behavior and bend ductility of Pt-30 wt. percent Rh-6 wt. percent W, Pt-26 wt. percent Rh-8 wt. percent W, and Pt-30 wt. percent Rh-lO wt. percent W, sheet samples of the alloys were cold rolled 35% and then annealed one hour in the temperature range 400 to 1370 C. in vacuum. The hardness data in Table II show that at temperatures above 400 C., the hardness decreases steadily with temperature. The eifect of annealing treatment on ductility was determined by bending the annealed samples to about 90 and then examining the sample metallographically. The results, as summarized in Table II, indicate that the samples which were annealed at 400 C. and 600 C. failed in the bend test. The cracked samples also exhibited an increased hardness with respect to the unannealed sample. The reason for this is unknown but perhaps attributable to the migration of impurities.

The samples were also examined metallographically to determine the recrystallization temperature. From this test, the recrystallization temperature was found to be between 980 and 1050 C.

of the Pt-Rh-W alloys increases with W level and this effect is more prominent at high temperatures. The ternary alloys are much stronger than the platinum-30 Wt. percent rhodium alloy at all temperatures. The Pt-30 wt. percent Rh-10 wt. percent W alloy is actually two and one-half times as strong as the binary alloy at 1316 C. The platinum, rhodium, tungsten alloys are also stronger than TZM to 1093 C. and are comparable to TZM but weaker than T-lll at 1316 C. The recrystallized alloys have excellent ductility at the 1316 C. which corresponds to impact temperature conditions on re-entry.

TAB LE III Ultimate tensile strength (p.s.i.)

Elongation (percent) Alloy composition Stress Recrystal- Stress Recrystal- (wt. percent) relieved lized b relieved e lized b Room temperature Pia-30 Rh 76,000 42 Pt-30 Rh-6 W 169,000 112, 000 15. 7 26 Pt-26 Rh-8 W 171,000

Pt-30'Rh-10 W 194, 000 118, 000 12. 5 14. 5

Pt-30 Rh 52, 000 48,000 23 28. 8 Pt-30Rl1-6W ,000 .5 .5 25 Pia-26 Rh-8 W 118,000

Pt- Rh-10 W 155, 000 TZM-.-. T-111 Pit-30 Rh 24,400 24,000 28 3 3O Pt-30 Rh-G W. 56,000 36,000 15 33.3 Pt-26 Rh-S W- 75,000 38, 000 12 18 Pt-30 Rh-10 W 89,000 000 9 15. 5 TZM 33,000 24 T-111 61,000 18 Pt-30 R11"... 10, 000 10,000 26. 5 26. 5 Pt-30 Rh-fi W- 19, 500 17, 500 51. 5 55 Pt-26 Rh-S W. 22, 500 21, 200 48 45 P530 Rh-IO W 25, 500 25, 500 50. 5 50. 5 TZM 2, 000 30 T-111 000 36 40 I Stress relieved at l,000 C.

b Recrystallization at 1,200 0.

EXAMPLE V The oxidation resistance of platinum-rhodium-tungsten alloys was determined by a thermal gravitational analysis in air. Tests were made at 760, 1000 and 1200 C. For comparison purposes, similar data for pure platinum, mo-

TABLE II Microhardness (DPH) Recrystallization 90 bend test Anneahng temp.

( 6% W 8% W 10% W 6% W 8% W 10% W 6% W 8% W 10% W 410 422 490 0 0 0 No crack No crack No crack. 465 467 512 0 0 0 Cracked Cracked Cracked. 422 439 482 0 0 0 No crack -.;-...-do Do. 390 410 453 0 0 0, do No crack No crack. 392 400 456 0 0 0 .d D0. 380 380 438 0 0 D0. 360 366 428 0 0 D0. 217 240 241 -100 -100 D0. 210 234 238 100 100 Do. 212 236 236 100 100 D0. 200 207 222 100 100 Do. 188 209 217 100 100 D0.

EXAMPLE IV In order to determine the mechanical properties ofthe alloys of this invention, two sets of sheet tensile specilybdenum and tungsten are also included in Table IV. No visible oxide layer was observed at 1000 and 1200 C., while a black oxide layer gradually appeared on the specimen oxidized at 760 C. after 200 hours. The oxidized specimens showed weight gain at 760 C. and weight losses at 1000 C. and 1200 C. due to evaporation of noble metal oxides. The oxidation rates of the alloys are higher than that for pure platinum by only a factor of about 4 but lower than those for molybdenum and tungsten by 5 to 6 orders of magnitude. Therefore, the oxidation resistance of the alloys is concluded to be excellent.

specimens. The heating operation, furthermore, caused no melting at the interface.

TABLE VI Heat treatment Reaction Harness (DPH) zone Temp. thickness Reaction Composite C.) Time (microns) Matrix zone Remarks PtRh-W/W 1,600 No interaction.

900 11 No interface melting. 1,100 63 620 Do. 1,100 67 799 Do. 1,600

PtRh-W/Mo 1, 600 min 0 No interaction.

900 1,000 hrs 10 No interface melting. 1 900 1,000hrs.,10 min. 11 Do.

,000 1,100 1,000 hrs 46 200 750 Do. 1,100 1,000 hrs.,10min 35 203 730 Do. 1, 000

TABLE Iv EXAMPLE VIII Gas tungsten-arc welding was used to evaluate the Alloy (wt. percent) 1,200o. 1,000o. 760C. weldability of the platinum, 26 wt. percent rhodium, 8 2 wt. percent tungsten alloy. Full penetration bead Welds Pt d'itii-eiv: n -5.2 0.5 "$0.18 were made on 20-mil sheet specimens in air and in an 528 ggf :gg :5? 135g inert gas chamber. The welded sheet showed no cracking Ta IIII III +250,000 +150,000 +9,o0o as Welded or after bending 90 when examined by dye M0 33001000 11500000 700, 000 penetrant and metallographic techniques.

EXAMPLE VI As can be seen from the foregoing examples, the alloy To simulate re-entry heating conditions, platinum, Wt. percent rhodium, and 9 and 10 wt. percent tungsten were heated in contact with T-111 in the temperature range 1595 to 1700" C. For comparison purposes, similar data for platinum, 30 wt. percent rhodium are shown in the summarized results of Table V. It should be noted that the platinum-30 wt. percent rhodium alloy shows interface melting at 1650 C., while the alloy of this invention with tungsten additions shows no such melting. It should be further noted that the tungsten addition is effective in reducing the reaction between the cladding alloy and the T-111, since the 10 wt. percent tungsten alloy has only a 90 micron reaction zone as compared to 140 microns for the 9 Wt. percent tungsten alloy. An additional 1000- hour test was conducted on the 9 WI. percent tungsten at 900 C. The results of this test were surprisingly superior to any results which could have been anticipated in that there was no reaction.

The compatibility of the platinum, 26 wt. percent rhodium, 8 wt. percent tungsten alloy with tungsten and molybdenum was determined by chemical vapor depositing a 9-mil layer of each metal onto 30-mil sheet samples of the alloy of this invention. The composite was then heat-treated at difierent temperatures and times. The results are shown in Table VI. The composites showed no signs of interaction or melting after 10 minutes at 1600 C. Due to the perfect bond between the alloy and CVD coatings, interdiifusion occurs and a narrow reaction zone is observed to form at the interface after contact for 1000 hours. However, metallographic examination showed no void formation, cracking or grain boundary interaction in either the matrix or reaction zone for all the composite of this invention is refractory, resistant to oxidation and has elevated temperature strength and ductility. Furthermore, the alloy of this invention is compatible with other refractory metals and alloys. Small amounts of other metals may also be added to the alloy of this invention to enhance certain properties. For example, the addition of 0.1 to 0.5 wt. percent titanium will improve the intermediate temperature ductility. A small amout of hafnium (0.2 to 1.0 wt. percent) may also be added to enhance the high temperature strength of the alloy. It is apparent that many minor deviations may be made while staying within the scope of the appended claims. For example, small amounts of impurities, such as those in Table I, are meant to be included within the scope of the claims. Many uses of this alloy, other than as an encapsulation, become apparent after reviewing the properties thereof. For example, this alloy possesses resistance properties such that it Would make an excellent electrical resistance heating element.

What is claimed is:

1. An alloy composition comprising 25 to 30 wt. percent rhodium, 6 to 10 wt. percent tungsten with the balance being platinum.

2. The alloy of claim 1 consisting essentially of 26 wt. percent rhodium, 8 wt. percent tungsten with the balance being platinum.

3. The alloy of claim 1 further comprising 0.1 to 0.5 Wt. percent titanium.

4. The alloy of claim 1 further comprising 0.2 to 1.0 wt. percent hafnium.

References Cited UNITED STATES PATENTS 1,978,198 10/1934 Handforth -172 X 2,406,172 8/1946 Smithells 75-172 X 2,636,819 4/1953 Streicher 75172 3,622,289 11/1971 Hansen et a1. 75172 X L. DEWAYNE RUTLEDGE, Primary Examiner E. L. WEISE, Assistant Examiner US. Cl. X.R. 250-106 S 

